Description of the Related Art
In an automotive vehicle, the wheels must be supported by a suspension apparatus so as to rotate freely. Also, in order to control an anti-lock braking system (ABS) or a traction control system (TCS), the rotational speed of the wheels must be detected. Rolling bearing units having a rotational speed detection device have heretofore been proposed, as disclosed for example in Japanese Utility Model First Publication KOKAI Nos. 3-128856, 5-4021 and in U.S. Pat. No. 5,063,345. A schematic diagram of the rolling bearing unit of U.S. Pat. No. 5,063,345 is shown in FIGS. 1 and 2.
The rolling bearing unit has a tone wheel or pulser ring 1 a part of which is illustrated in FIG. 1 and a stationery or fixed sensor 3 which is mounted on a suspension apparatus. The tone wheel 1 is generally made of a magnetic material and is ford with gear-shaped serrations 2 on an outer peripheral surface thereof. The tone wheel 1 is associated with a vehicle wheel through the bearing unit so as to rotate together with the vehicle wheel. The fixed sensor 3 comprises a permanent magnet 4 which produces an axially aligned (top-bottom direction in FIG. 1) magnetic field, a yoke 5 made of a magnetic material, end a coil 6 wound around the yoke 5.
The yoke 5 has a base end 5a (upper end face in FIG. 1) and a tip end 5b (lower end face in FIG. 1). The base end face 5a of the yoke 5 is abutted against one end face of the permanent magnet 4 (lower end face in FIG. 1 ), while the tip end face 5b of the yoke 5 faces towards the serrations 2. As a result, the magnetic field is formed around the permanent magnet 4 and the yoke 5 as shown by the dotted lines with arrows in FIG. 1.
The density of a magnetic flux producing the magnetic field becomes higher when a protrusion 2a of the serrations 2 is opposed to the tip end face 5b of the yoke 5, and becomes lower when a recess 2b of the serrations 2 is opposed to the tip end face 5b of the yoke 5. The induced electromotive force in the coil 6 thus varies as shown in FIG. 2, in accordance with the variation in magnetic flux density of the magnetic field formed around the coil 6. Since the frequency of the induced electromotive force is proportional to the rotational speed of the vehicle wheel (not shown), then the ABS or the TCS can be appropriately controlled by inputting the induced electromotive force obtained as an output signal from the sensor 3, to a controller 7 of the ABS or the TCS.
With the conventional rolling bearing unit fitted with the rotational speed detection device constructed and operated as mentioned above, there are the following problems.
Namely, in the support section for the rotating wheel, it is not always possible to maintain enough space inside for installation of the sensor 3. It is thus necessary to miniaturize the sensor 3. However mere miniaturization of the sensor 3 causes not only a drop off in output (the difference V in FIG. 2 between the maximum and minimum voltages) of the sensor 3, but also a loss in wheel speed detection reliability.
This is because, in order to miniaturize the sensor 3, it is necessary to make the yoke 5 smaller in size. However, to sufficiently maintain the magnitude (voltage) of the output signal from the sensor 3, a permanent magnet of high magnetic strength or high magnetic flux density must be used for the permanent magnet 4. As a result, there is the likelihood of saturation of the magnetic flux inside the yoke 5. If this occurs, the variation in magnetic flux density between the condition wherein the tip end face 5b of yoke 5 is opposed to a protrusion 2a of the serrations 2 and the condition wherein it is opposed to a recess 2b of the serrations 2 is reduced, so that the output becomes extremely small.
In this respect, a rolling bearing unit fitted with a rotational speed detection device incorporating a sensor which avoids saturation of the magnetia flux inside the yoke while still enabling installation in a limited space, and providing a sufficient output, is disclosed for example in Japanese Utility Model First Publication KOKAI No. 5-4021. A schematic diagram of this bearing unit is shown in FIGS. 3 and 4.
The construction disclosed in this publication has a tone wheel or pulser ring 1 and a permanent magnet 8 which has a magnetic field directed in a radial direction of the tone wheel 1. A projection 9 is provided on an inner peripheral face of the permanent magnet 8 while a yoke 10 made of a magnetic material is provided with a central portion thereof in abutting contact with the outer peripheral face of the permanent magnet 8 and with both end portions 11 turned down.
The projection 9 together with the end portions 11 are arranged so as to be simultaneously faced to protrusions 2a of the serrations 2 on the outer periphery of a tone wheel 1.
A coil 12 which is wound around the yoke 10, comprises a first coil portion 12a and a second coil portion 12b wound in opposite directions to each other and connected together in series.
With such a construction, the magnetic flux from the outer side face of the permanent magnet 8 flows in two separate systems inside the yoke 10 as shown by arrows. Therefore, compared to the structure of FIG. 1, the magnetic flux is less likely to become saturated inside the yoke 10. However there are still problems with obtaining a reliable effect. If the cross-sectional area of the yoke 5 of FIG. 1 is assumed to be the same as that of the yoke 10 of FIGS. 3 and 4, then the structure of FIGS. 3 and 4 can take up to approximately two times the magnet flux density of the structure of FIG. 1. However if a permanent magnet 8 having a magnet flux density in excess of this level is used, the output of the sensor 13 will still drop considerably with saturation of the magnetic flux in the yoke 10.
With the construction of FIGS. 3 and 4, all of the magnet flux utilized in detecting rotational speed passes through the space between the tip end face 9 of the permanent magnet 8, and the tip of the protrusion 2a of the sarrations 2 which are opposed to the tip end face 9 of the permanent magnet 8. However, in order to accurately detect rotational speed, it is necessary to make the pitch of the serrations 2 fine, so that the area of the tips of the protrusions 2a, and the cross sectional area of the space is reduced. Therefore when the magnetic flux density of the permanent magnet 8 is increased, the magnetic flux would become saturated in this space, so that there is still a considerable drop in output from the sensor 13.